New all-electrostatic vidicon

A new, all-electrostatic vidicon has been developed, in which crossed planar lenses are used to focus the electron beam sequentially in the frame and line directions, instead of the rotationally symmetrical lenses used in present-day electrostatic vidicons. The arrangement is the equivalent of crossed cylindrical lenses in light optics. Such planar lenses can be formed between two pairs of parallel plates. The second pair of plates of each lens is also used for scanning. By introducing a fine mesh in the plane of symmetry of a 2-element lens, one pair of plates may be omitted, thus increasing the focusing power of the lens and making it shorter. A computer program was written to plot electron trajectories. The computed results, together with experiments made on a scaled-up demountable gear and with sealed-off tubes, were used to optimise the design. In the course of the work, it was found that the image of a crossover, rather than that of a physical aperture in the electron gun, gave a focused spot more suitable for this particular application. The modulation depth of a 400 TV line pattern of black and white bars (i.e. 400 lines per picture height) is approximately 40% in the centre and 25% in the corners. Occasionally, over 50% modulation has been achieved in the centre. A remarkable feature is that the image field is practically flat. The geometrical fidelity is correct to within ±0.25% over the whole scanned area, is even better in the quality area in the centre, and is very uniform from one tube to another. This makes this type of tube highly suitable for the registration required in colour cameras.